It is known that in an automatic winder comprising a plurality of winding units arranged in parallel, one air conduit is disposed and used in common for supplying suction air streams used for performing the yarn knotting operation to the respective winding units and one blower is connected to said air conduit. In this automatic winder, the capacity of the blower is so large that even if the yarn knotting operation is performed simultaneously in all the winding units, the yarn knotting operation can be conducted normally. However, from experience, it has been confirmed that among 50 units, only 3 to 10 units require the yarn knotting operation simultaneously, and it seldom happens that all the capacity for 50 units is necessary. Accordingly, there was adopted a method in which a blower having a capacity for 10 units is used and in the winding units where the first yarn knotting operation is unsuccessful, the second yarn knotting operation is conducted with an intention that since the number of the winding units where the second knotting operation is required is reduced because th second knotting operation need not be performed in the winding units where the first knotting operation has been successful. This method, however, is defective in that when in more than 10 of winding units, the knotting operation is carried out simultaneously, it is impossible to operform the yarn knotting operation successfully in 10 or more of the winding units and the number of the winding units where the yarn knotting operation can be performed successfully is reduced, leading to wasteful consumption of the energy.
The applicant already proposed a method in which two blowers having a small capacity are connected, one blower is always used and only reduction of the pressure in the air conduit is detected, the second blower is operated. However, this method is defective in that a certain time is required for imparting the effect of the second blower to the respective winding units through the air conduit, and from the viewpoint of the running cost, the method is disadvantageous in that a motor which is seldom used should be disposed.